Ultra-broadband low-noise gain-flattened rare-earth-doped fibre amplifier

ABSTRACT

An optical amplifier waveguide provides variable amplification gain to a broad wavelength input signal. The degree of amplification is proportional to the length of the transmission of the input signal along the amplification waveguide. The amplification waveguide means further comprises a series of output wavelength couplers, which are preferably fiber gratings, placed along the length of the amplification waveguide for coupling a predetermined amplified signal from the amplification waveguide to the output fiber. Additionally, the optical amplifier waveguide of the present invention may include a plurality of noise wavelength coupling means, which are preferably fiber gratings, for coupling unwanted noise from the amplification waveguide means to a noise dissipation means.

FIELD OF THE INVENTION

The present invention relates to amplification of optical signals and inparticular those signals transmitted through optical fibres andamplified utilising a rare-earth doped fibre amplifier.

BACKGROUND OF THE INVENTION

Recently, the utilisation of optical fibres for communications hasbecome increasingly popular due to their high bandwidth capabilities.The wavelengths normally utilised for optical fibre transmission havebeen traditionally related to the low attenuation areas of thetransmission spectrum of a single mode optical fibre. Turning initiallyto FIG. 1, there is illustrated the spectrum of a typical attenuationrate for single mode optical fibres. The figure indicates two particularwindows of interest for low loss transmission, the first being atapproximately 1550 nm and the second at 1310 nm. The window at 1550 nmhas become particularly popular for its low attenuation rate.

Recently the all optical rare-earth doped fibre amplifiers have alsobecome increasingly popular for providing for the all opticalamplification of an input signal. One particular form of amplifier inpopular use is the Erbium doped fibre amplifier (EDFA) which hasparticularly strong amplification also in the 1550 nm region. FIG. 2illustrates an example of the gain provided by a standard EDFA fordifferent pumping powers (normalised to one with the gain alsonormalised to one). As can be seen from FIG. 2, the gain profile of anEDFA is highly irregular. In the past, when only a single wavelength istransmitted by an optical fibre, this is not a problem. However,recently wavelength division multiplexed (WDM) systems have beenproposed and constructed with, as the name suggests, the optical fibrecarrying many different channels at different frequencies orwavelengths. Unfortunately, the amplification profile of an EDFA resultsin each channel experiencing a substantially different gain and hence aWDM system is likely to be problematic for amplification by a EDFAamplifier unless the gain profile can be held to be substantiallyconstant. It will, of course, be noted from FIG. 2 that an EDFA normallyprovides a degree of useable gain across a broad spectrum of suitablewavelengths however, as can be clearly seen from FIG. 2, the gainspectrum is “swamped” by the central peak.

SUMMARY OF THE INVENTION

In accordance with a first aspect of the present invention there isprovided an optical amplifier for a broadband signal comprising anamplification waveguide having a gain per length of transmission thatdepends on the wavelength of a signal to be amplified; a first pluralityof first coupling means positioned along the length of the amplificationwaveguide, each coupling means arranged to couple light of an associatedselected wavelength from the amplification waveguide to an output meansof the optical amplifier; and wherein the respective coupling means arepositioned so that the gain experienced in the amplification waveguideis substantially equal for the different associated wavelengths.

The first plurality of first coupling means may be formed within theamplification waveguide.

The first plurality of first coupling means may alternatively be formedwithin the output means.

The output means may comprise an output waveguide.

The first coupling means may each comprise an optical grating. Thegrating may comprise a long period plating.

The amplification waveguide may comprise the core of an optical fibre.

In one embodiment, the output means is formed in the cladding of theoptical fibre.

The output means may comprise a further core of the optical fibre.

The amplification waveguide may be formed from a rare earth-doped glass.The rare earth element may be erbium.

The optical amplifier may further comprise a second plurality of secondcoupling means positioned along the length of the amplificationwaveguide, each coupling means arranged to couple light of an associatedselected wavelength from the amplification waveguide to a noisedissipation means; and the respective second coupling means arepositioned after a corresponding one of the first coupling means havingthe same associated wavelength, so that residual signal of the samewavelength is coupled from the amplification waveguide to the noisedissipation means.

The noise dissipation means may comprise a waveguide.

The second coupling means may comprise a grating.

The second coupling means may be formed in the amplification waveguide.Alternatively, the second coupling means may be formed in the noisedissipation means.

The second coupling means may further be arranged to couple other noisefrom the amplification waveguide into the noise dissipation means.

In accordance with a second aspect of the present invention there isprovided a method of optically amplifying a broad wavelength signalcomprising the steps of amplifying the broad wavelength signal in anamplification waveguide having a gain per length of transmission thatdepends on the wavelength of a signal to be amplified; coupling aplurality of components of the broad wavelength signal from theamplification waveguide to an output means; and wherein each of thecomponents is coupled from the amplification waveguide at a positionsuch that the gain experienced in the amplification waveguide issubstantially equal for the different components.

The method may further comprise the step of coupling a plurality ofcomponents of the broad wavelength signal from the amplificationwaveguide to a noise dissipation means; and wherein each component iscoupled from the amplification waveguide to the noise dissipation meansat a position after the position at which a component of the samewavelength is coupled from the amplification waveguide to the outputmeans to couple residual signal of that wavelength from theamplification waveguide.

In accordance with a third aspect of the present invention there isprovided an optical fibre having a plurality of cores, the cores havingdiffering respective propagation constants at a predeterminedwavelength, wherein a long period grating is provided in at least one ofthe cores, the long period grating being configured such that, in use,it matches the propagation constant of said core at the predeterminedwavelength to the propagation constant of another one of the cores forcoupling of light from said core into the other core.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of thepresent invention, preferred forms of the invention will now bedescribed, by way of example only, with reference to the accompanyingdrawings in which:

FIG. 1 illustrates the typical attenuation rate for single mode fibres;

FIG. 2 illustrates a graph of amplification gain for an erbium dopedfibre amplifier (EDFA) for different pumping levels;

FIG. 3 illustrates in schematic form a first illustrative arrangementdiscussed with reference to the preferred embodiment; and

FIG. 4 illustrates, in schematic form, one form of the preferredembodiment of the present invention.

DESCRIPTION OF PREFERRED AND OTHER EMBODIMENTS

In the preferred embodiment an erbium doped fibre amplifier is utilisedin conjunction with a grating system to transfer the signal associatedwith each wavelength from an input channel to an output channel at aposition determined by the expected gain on the input signal at aparticular wavelength. Subsequently, the amplification of noiseassociated with the input signal of the same wavelength is discarded or“dumped” to a dispersion or noise channel for dissipation. This processis repeated for each signal of interest.

In order to obtain a clear understanding of the preferred embodiment, aninitial apparatus is proposed and discussed with reference to FIG. 3. Inthis design, an input signal 10 is fed to an Erbium doped amplifyingcore 11 which is placed in close proximity to a second non-amplifyingcore 12. The two cores 11, 12 exist within a cladding layer 13.Normally, whilst the two waveguides 11, 12 are placed in close proximityhowever, they are designed such that coupling between them does notoccur (eg. by choice of core diameter and refractive index, which whilstproviding similar cutoff wavelengths as needed gives differingpropagation constants). The waveguide 11 can comprise an EDFA which isdoped and pumped strongly. It potentially provides gain across a verylarge spectrum. However, as noted previously, the strong gain in thepeak of the gain spectrum of FIG. 2 swamps the tails. There is a pointalong the length of the waveguide 11 where any wavelength within thegain band reaches a certain level of gain (say 30dB), those wavelengthsat the gain peak experience this overall level of gain at the first partalong the length of the amplifier waveguide 11 and those in the tailpart of the gain band experience the level of gain in the last part ofthe waveguide 11. The gain flattening is achieved by coupling the twowaveguides 11, 12 together at a certain point eg. 15-17 dependant on theamount of gain experienced at that point for the particular wavelength.The coupling can be provided by means of a grating written into thefibre. Preferably, long period gratings are utilised. The frequency ofthe grating written will be dependant upon the desired couplingwavelength. The coupling at point 15 can be for the wavelength receivingmaximum gain near the gain peak whereas the coupling at the point 17 canbe for the wavelength experiencing a much lower level of gain near thegain tail along the waveguide 11. Hence, the coupling point varies downthe length of the fibre according to the wavelength. The position of thegratings eg. 15-17. being adjusted such that the output gain of theoutput signal 18 is substantially the same for all wavelengths, therebyachieving gain flattening.

Unfortunately, the arrangement of FIG. 3 does not take into account anynoise or residual signal that, for example, is not coupled out of thegrating 15. Unfortunately, down the subsequent length of the waveguide11, the residual noise (and signal which was supposed to be coupled outat the point 15) will be amplified dramatically thereby absorbing energywithin the waveguide 11 which would otherwise be utilised to amplifyother wavelengths. With the very high gains in the channel 11 at thegain peak, spontaneous emissions will potentially experience sufficientamplification to cause lasing. However, the device of FIG. 3 may stillbe suitable in its own right.

Turning now to FIG. 4, there is illustrated an alternative more suitablearrangement 20. This arrangement is similar to that illustrated in FIG.3 and includes broadband input signal 21 and amplified output signal 22.An EDFA amplifier is provided 23 and pumped in the usual manner. Takingthe example of one wavelength only, a first coupling is provided 24 forcoupling the signal to output waveguide 25 as previously described.Along the rest of the fibre a series of gratings eg. 26 are provided forcoupling any residual signal and amplified noise from the waveguide 23to a noise dissipation waveguide 27 with the grating 26 beingparticularly tuned so as to provide for coupling directly to the noisedissipation waveguide 27. If desired, the noise dissipation waveguide 27can be the cladding of the fibre. The waveguide 27 provides a convenientplace to dump the amplified noise before it can accumulate and depletethe gain or cause lasing. In the simple case, as noted previously, thewaveguide 27 can comprise the cladding. Alternatively, it could be adoped waveguide which is unpumped as a means for actively removing thenoise. A series of gratings 26 canoe provided along the waveguide 23after a first coupling waveguide 24 so as to minimise any amplificationof the noise signals remaining after coupling to the output 25.

The arrangement 20 of FIG. 4 illustrates, for clarity, the processing ofone wavelength only, with overlapping gratings being written so as todeal with other wavelengths so as to provide for both flat gain couplingto waveguide 25 in addition to noise dumping to waveguide 27. Hence, thepreferred embodiment can include a multi-core fibre with couplingprovided by overlapping long period gratings. The coupling to thewaveguide 25 can occur at each point where the requisite wavelengthreaches a certain gain with immediately after the coupling to the outputchannels 25 all subsequent noise associated with the output frequencybeing coupled to the “noise dumping” channel 27.

Of course, other arrangements are possible: For example, counterdirectional pumping might be utilised so as to provide for maximum tailgain; Codirectional and Bidirectional pumping are also possible.Alternatively, a fourth core waveguide could be introduced withappropriate gratings for distribution of the pumping energy inaccordance with the needs. In other arrangements gratings can be writtenin the other waveguides. For example, the gratings could be written tothe output and noise waveguides instead of the gain/input waveguides.

In a further alternative arrangement, the noise channel can comprise thecladding. Other arrangements can include, for example, a central outputchannel around which is circumferentially arranged the gain amplifierand input channel and further around which is circumferentially arrangedthe noise channel. The channels can alternatively be in a differentorder.

The principles of preferred embodiment can further be extended to othertransmission windows. For example, in the 1300 nm window Nd³⁺in silicais known to provide gain in this region but is normally unusable becauseof strong competitive lasing combined with poor centring on the band ofinterest. If gain is provided across the whole band, then highefficiency outcoupling of problem parts of the spectrum may result inrealistic silica amplifiers for the 1300 nm region.

An extension to the preferred embodiment permits efficient pumping of again medium from a multimode source. A multimode pump can be launchedinto a multimode core which runs close to a gain core. The cores aredesigned such that as far as is possible there is no coupling betweeneither the signal or pump modes of the gain core and any of the modes ofthe pump core. Gratings are introduced to couple modes of the multimodepump to the fundamental mode of the pump in the gain channel. Thespacing of the gratings along the fibre can be such as to allow forstrong absorption of those components of the pump already coupled (sothat there is negligible back-coupling). In this manner, each mode maybe coupled over and absorbed. This has an advantage over existingschemes such as cladding pumping, where the absorption of the pumprelies on a simple overlap and the efficiency scales with the areas ofthe waveguides. This scheme may be particularly advantageous for lasersas well as amplifiers.

Further, it is known that long period gratings are highly sensitive toperturbations. This can be used to effect tuning of the coupling andhence the overall performance. For instance, first order corrections forspectral gain variations due to changes in inversion from changes insignal or pump power could be corrected by, for instance strain ortemperature, tuning of the long period gratings.

It would further be appreciated by a person skilled in the art thatnumerous variations and/or modifications may be made to the presentinvention as shown in the specific embodiments without departing fromthe spirit or scope of the invention as broadly described. The presentembodiments are, therefore, to be considered in all respects to beillustrative and not restrictive.

We claim:
 1. An optical amplifier for a broadband signal comprising: anamplification waveguide having a gain per length of transmission thatdepends on the wavelength of a signal to be amplified; a first pluralityof first coupling means positioned along the length of the amplificationwaveguide, each coupling means arranged to couple light of an associatedselected wavelength from the amplification waveguide to an output meansof the optical amplifier, the output means comprising an outputwaveguide for transmission of an amplified signal; and wherein therespective coupling means are positioned so that the gain experienced inthe amplification waveguide is substantially equal for the differentassociated wavelengths.
 2. An amplifier as claimed in claim 1, whereinthe first plurality of first coupling means are formed within theamplification waveguide.
 3. An optical amplifier as claimed in claim 1,wherein the first plurality of first coupling means are formed withinthe output means.
 4. An optical amplifier as claimed claim 1, whereinthe first coupling means each comprise an optical grating.
 5. An opticalamplifier as claimed in claim 4, wherein the grating comprises a longperiod grating.
 6. An amplifier as claimed in claim 1, wherein theamplification waveguide comprises the core of an optical fiber.
 7. Anamplifier as claimed in claim 6, wherein the output means is formed inthe cladding of the optical fibre.
 8. An amplifier as claimed in claim7, wherein the output means comprises a further core of the opticalfiber.
 9. An amplifier as claimed in claim 1, wherein the amplificationwaveguide is formed from a rare earth-doped glass.
 10. An amplifier asclaimed in claim 9, wherein the rare earth element is Erbium.
 11. Anoptical amplifier as claimed in claim 1, further comprising a secondplurality of second coupling means positioned along the length of theamplification waveguide, each coupling means arranged to couple light ofan associated selected wavelength from the amplification waveguide to anoise dissipation means; and wherein the respective second couplingmeans are positioned after a corresponding one of the first couplingmeans having the same associated wavelength, so that a residual signalof the same wavelength is coupled from the amplification waveguide tothe noise dissipation means.
 12. An amplifier as claimed in claim 11,wherein the noise dissipation means comprises a waveguide.
 13. Anamplifier as claimed in claim 12, wherein the second coupling meanscomprises a grating.
 14. An amplifier as claimed in claim 12, whereinthe second coupling means are formed in the amplification waveguide. 15.An amplifier as claimed in claim 12, wherein the second coupling meansare formed in the noise dissipation means.
 16. An amplifier as claimedin claim 12, wherein the second coupling means are further arranged tocouple a noise signal from the amplification waveguide to the noisedissipation means.
 17. An amplifier as claimed in claim 11 wherein thesecond coupling means comprises a grating.
 18. An amplifier as claimedin claim 17, wherein the second coupling means are formed in theamplification waveguide.
 19. An amplifier as claimed in claim 17,wherein the second coupling means are formed in the noise dissipationmeans.
 20. An amplifier as claimed in claim 17, wherein the secondcoupling means are further arranged to couple a noise signal from theamplification waveguide to the noise dissipation means.
 21. An amplifieras claimed in claim 11, wherein the second coupling means are formed inthe amplification waveguide.
 22. An amplifier as claimed in claim 21,wherein the second coupling means are further arranged to couple a noisesignal from the amplification waveguide to the noise dissipation means.23. An amplifier as claimed in claim 11, wherein the second couplingmeans are formed in the noise dissipation means.
 24. An amplifier asclaimed in claim 23, wherein the second coupling means are furtherarranged to couple a noise signal from the amplification waveguide tothe noise dissipation means.
 25. An amplifier as claimed in claim 11,wherein the second coupling means are further arranged to couple othernoise signal from the amplification waveguide to the noise dissipationmeans.
 26. A method of optically amplifying a broad wavelength signalcomprising the steps of: amplifying the broad wavelength signal in anamplification waveguide having a gain per length of transmission thatdepends on the wavelength of a signal to be amplified; coupling aplurality of components of the broad wavelength signal from theamplification waveguide to an output means, the output means comprisingan output waveguide for transmission of an amplified signal; and whereineach of the components is coupled from the amplification waveguide at aposition such that the gain experienced in the amplification waveguideis substantially equal for the different components.
 27. A method asclaimed in claim 26, further comprising the step of coupling a pluralityof components of the broad wavelength signal from the amplificationwaveguide to a noise dissipation means; and wherein each component iscoupled from the amplification waveguide to the noise dissipation meansat a position after the position at which a component of the samewavelength is coupled from the amplification waveguide to the outputmeans to couple a residual signal of that wavelength from theamplification waveguide.
 28. An optical amplifier for a broadband signalcomprising: an amplification waveguide having a gain per length oftransmission that depends on the wavelength of a signal to be amplified;and a plurality of first couplers positioned along the length of theamplification waveguide, each first coupler arranged to couple light ofan associated selected wavelength from the amplification waveguide to anoutput waveguide for transmission of an amplified signal; wherein therespective first couplers are positioned so that the gain experienced inthe amplification waveguide is substantially equal for the differentassociated wavelengths.
 29. An optical amplifier as claimed in claim 28,wherein the first couplers comprise an optical grating.
 30. An opticalamplifier as claimed in claim 29, wherein the grating comprises a longperiod grating.
 31. An optical amplifier as claimed in claim 28, furthercomprising a plurality of second couplers positioned along the length ofthe amplification waveguide, each second coupler arranged to couplelight of an associated selected wavelength from the amplificationwaveguide to a noise dissipator; and wherein the respective secondcouplers are positioned after a corresponding one of the first couplershaving the same associated wavelength, so that a residual signal of thesame wavelength is coupled from the amplification waveguide to the noisedissipator.